This invention relates generally to nuclear reactors and more particularly to an auxiliary heat removal system for use in gas-cooled nuclear reactors.
A nuclear reactor produces heat by the fissioning of a fissile material which is fabricated into fuel elements and assembled into a nuclear core. In gas-cooled nuclear reactors, the heat produced by the fissile materials is transferred to an inert gas such as helium or argon, which is then circulated, typically, through turbines, heat exchangers, and compressors before being returned to the nuclear core. The power output of the turbines is then converted into electrical power.
The use of a nuclear reactor as a heat source introduces the need for a cool-down system to remove, residual heat. A nuclear reactor does not cease generating heat immediately upon being shut down. Delayed neutrons continue to cause fission to occur for a period of time after shutdown. For a longer period of time, dependent upon the reactor characteristics and prior operating history, the products resulting from the fission of the fuel continue to decay, and in decaying release energy. Additionally, the fuel elements themselves are often operated at temperatures above the level at which structural damage would occur if cooling were not provided. These abovementioned factors mandate the requirement that adequate cooling be provided for the nuclear core after the reactor is shut down, to remove the residual heat retained in the core. Typically, cool-down under normal conditions, such as for refueling, is achieved through the use of the normal circulatory system. The turbines, heat exchangers, and compressors continue operations until the desired temperature level of the nuclear core is attained. In the unlikely event of a emergency condition, it may not be possible for the normal circulatory system to remove the heat generated by the nuclear core. For example, one of the emergency conditions which is postulated, although its probability is extremely small, is the complete failure of the normal circulatory system to operate. In the event of such an occurrence, a separate system must be provided to remove the heat generated by the nuclear core after it has been shut down.
In the prior art, at least two different systems have been proposed to remove the residual heat retained after reactor shut-down. One system involves installing a redundant system to provide circulation whenever the normal circulatory system is inoperable. This emergency system generally is dependent upon an external power source and external controls. As such, these external controls and the external power source must be operable under all possible emergency conditions, a difficult and expensive requirement.
Another method utilized in the prior art is to remove the residual heat through convection circulation of the reactor coolant. Although this system removes the dependence upon an external power source, it has disadvantages. The natural convection circulation may not be adequate to remove sufficient heat from the nuclear core. Additionally, the use of the natural convection circulation system is dependent upon the reactor gravitational orientation.